A screen reader opens your PDF and encounters an image. If that image has alternative text, or alt text, the screen reader speaks the description aloud and the listener understands what the image conveys. If the image has no alt text, the screen reader might say "Image" and move on, leaving the listener to guess what they missed. In the worst case, the screen reader reads the image's auto-generated filename instead: "img_20251007_1432_final_v3.png," which conveys nothing useful.
Alt text is the invisible bridge between visual content and the text-based interface of assistive technology. Adding it to images in a PDF is a straightforward mechanical task, but writing good alt text, descriptions that are concise, meaningful, and appropriate to the document's purpose, requires judgment. This article covers both the how and the what: how to add alt text to any image in a PDF using common tools, and what makes alt text genuinely useful rather than technically compliant but practically worthless.
How to Add Alt Text to Images Using Adobe Acrobat Pro
Adobe Acrobat Pro provides the most direct path for adding and editing alt text on PDF images. Open the PDF, go to the Accessibility tools, and select Set Alternate Text. Acrobat walks you through every figure element in the document one by one, displaying each image and prompting you to enter a description. This wizard-style workflow ensures you do not miss any images. For each image, you can type a description directly or mark it as a decorative artifact if it conveys no information and should be skipped by screen readers.
You can also add alt text manually through the Tags panel. Expand the document's tag tree, find the Figure tag corresponding to the image, right-click it, and select Properties. The Object Properties dialog includes an Alternate Text field. This method is slower than the wizard but gives you full control over which elements receive alt text and which are marked as artifacts. It is the right approach when you need to edit alt text on a single image rather than walking through every figure in the document.
How to Add Alt Text Using Browser-Based Tools
A dedicated PDF Editor includes an accessibility inspection mode that identifies images in the PDF and lets you add or edit alt text for each one. Upload the file and open the accessibility panel. The tool scans the document, lists every image element, and shows the current alt text status for each. Images with no alt text are flagged. Click on any image to open an editing field where you type the description. When you save and download the file, the alt text is embedded in the PDF's tag structure, ready for screen readers to extract.
The browser-based approach works well for PDFs that already have a tag structure. For scanned PDFs or files where images were not tagged during creation, the browser tool may not detect images as distinct figure elements because the tag tree does not exist. In that case, running OCR and auto-tagging before adding alt text is the necessary first step. WukongPDF's OCR tool generates a tag structure from scanned content, which then makes individual image elements accessible for alt text editing.
Writing Alt Text That Actually Helps the Reader
Adding alt text is the mechanical task. Writing good alt text is the skill. The most common mistake is describing what the image looks like rather than what it communicates. For a company logo, "Blue rectangle with white text reading Acme Corporation" is a visual description. "Acme Corporation logo" conveys the same information in four words. The screen reader user does not need to know the logo's color scheme unless color is relevant to the document's meaning.
For charts and graphs, the alt text should summarize the key finding, not describe every data point. A bar chart comparing quarterly revenue might have alt text that reads "Bar chart showing quarterly revenue increased from $2.1M in Q1 to $3.4M in Q4, a 62% annual increase." The screen reader user gets the insight the chart was included to convey, without the cognitive load of parsing four quarters of exact values narrated linearly. If the exact data matters, include a data table alongside the chart and reference it in the alt text: "See Table 3 for quarterly breakdown."
For decorative images, images that add visual interest but no information, the correct alt text is no alt text at all. Mark the image as an artifact in the tag tree so screen readers skip it entirely. Describing a decorative image wastes the listener's time and attention. A good rule of thumb: if you would not mention the image in a phone call describing the document to someone, it does not need alt text.
Adding Alt Text at the Source: Word, InDesign, and Google Docs
The most efficient place to add alt text is in the source document before exporting to PDF. In Microsoft Word, right-click any image, select View Alt Text, and type the description in the panel that appears. This alt text travels with the image through the export process and is embedded in the resulting PDF's tag structure automatically. No post-export editing is needed. The same workflow applies to PowerPoint: right-click an image, select Edit Alt Text, and enter the description.
In Adobe InDesign, select an image frame, open Object Export Options, go to the Alt Text tab, and choose Custom from the Alt Text Source dropdown. Enter the description. InDesign exports this alt text to the tagged PDF. In Google Docs, right-click an image, select Alt Text, and type the description. Google Docs exports alt text to PDF automatically. The pattern is consistent across all major authoring tools: add alt text at the source, and the PDF inherits it. Authoring for PDF Accessibility from the beginning is always faster than remediating after export.
Verifying That Alt Text Is Present and Correct
After adding alt text, verify it before distributing the document. Run an accessibility check in Acrobat Pro, PAC, or WukongPDF's PDF Images accessibility scanner. The checker lists every image and whether it has associated alt text. But a checkmark next to an image only tells you that alt text exists. It does not tell you whether the text is good.
Read each alt text aloud. Does it make sense without looking at the image? Would it help someone who cannot see the page understand what the image contributes? If the alt text says "Chart" and nothing else, it technically passed the automated check but functionally failed the user. Automated checkers verify presence. Human review verifies quality. Both steps matter.
Enable a screen reader on your computer, VoiceOver on Mac, Narrator on Windows, and listen to your document. Pay attention to how the alt text sounds in context, surrounded by the surrounding paragraphs. A description that reads well in isolation might feel redundant or jarring when heard immediately after a caption that already says the same thing. Adjust the alt text so it complements the surrounding content rather than repeating it. The goal is a seamless listening experience where the alt text feels like a natural part of the narrative, not an interruption for technical compliance.